1. Field of the Invention
The present invention relates to a fixing device for use in an image forming apparatus such as a printer, a facsimile machine, a photocopier, etc., and more particularly to a temperature detecting structure for a heating mechanism in a fixing device.
2. Discussion of the Background
Generally, in an image forming apparatus such as a photocopier, a facsimile machine, a printer, etc, an unfixed toner image carried on a recording medium, for example a sheet, is fixed onto the recording medium by a fixing device. Then the recording medium having a fixed toner image thereon is discharged from the image forming apparatus as a copy sheet or a printed sheet.
A known fixing device employs a structure in which a pair of rollers are opposed to each other. One roller is a heating roller, and another roller is a pressing roller which press-contacts the heating roller. In this type of fixing device, a recording medium carrying an unfixed toner image is passed through a nip part between the heating roller and the pressing roller. While the recording medium passes through the nip part, a toner image on the recording medium is fused and fixed on the recording medium by heat and pressure.
Another type of fixing device employs a structure having a combination of rollers and a belt. In this structure, for example, a fixing belt is extended and stretched around a heating roller and a fixing roller. In addition, a pressing roller is arranged opposite to the fixing roller via the fixing belt.
The heating roller and the pressing roller include heat sources to heat back and front surfaces of the fixing belt, respectively.
Because the volume and the thermal capacity of the fixing belt are smaller than those of the roller, the temperature of the fixing belt can be increased in a short period of time. For this reason, the fixing belt has an advantage in that heating-up is quick upon start of the fixing device compared to the above-described structure of the fixing device having a heating roller and a pressing roller without a fixing belt. Further, the provision of the heat source in the pressing roller results in the acceleration of heating-up at both front and back surfaces of the fixing belt.
The above-described fixing belt has a two-layer structure. When each roller is made of aluminum having high thermal conductivity, the fixing belt includes a base member which contacts the surfaces of the rollers and is made of a conductive metallic member of high heat capacity such as stainless steel, etc. The fixing belt further includes a releasing layer made of silicone rubber or fluororesin on the surface of the base member.
In a background fixing device, the surface temperature of such a fixing belt is controlled to be at a predetermined value by detecting the surface temperature of the fixing belt with a temperature detecting member arranged in a non-contacting relation to the front surface of the fixing belt. Because the temperature detecting member is held in a non-contacting relation to the front surface of the fixing belt, the surface temperature of the fixing belt may not be detected with accuracy.
If the temperature detecting member is arranged in a contacting relation to the front surface of the fixing belt, the front surface of the fixing belt may be damaged by the temperature detecting member, thereby resulting in deterioration of image quality.
For the above-described reasons, as an alternative configuration, the temperature detecting member can be arranged in a contacting relation to the back surface of the fixing belt. However, the fixing belt is configured to be rotated during the rotations of the heating roller and the pressing roller, by frictional contact pressure between the fixing belt and the heating/pressing rollers. At the time of rotation start and stop of the heating/pressing rollers, the fixing belt may move by itself due to its inertia. Therefore, the contact surfaces of the fixing belt and the heating/pressing rollers, which have a smaller hardness than that of the other contact surfaces, may abrade due to frictional resistance. As a result, abrasion powder is typically produced between the contact surfaces of the fixing belt and the heating/pressing rollers.
When the temperature detecting member contacts the back surface of the fixing belt so as to detect the surface temperature of the fixing belt, the above-described abrasion powder may enter a space around the contact surface of a temperature detecting portion of the temperature detecting member and the fixing belt. Due to the entry of the abrasion powder, the temperature detecting portion typically abrades.
In addition, the abrasion of the temperature detecting portion is typically caused by friction between the temperature detecting portion and the fixing belt.
The abrasion of the temperature detecting portion may cause the contact condition of the temperature detecting member and the fixing belt to be unstable. As a result, the temperature detecting member cannot detect the surface temperature of the fixing belt with accuracy, so that the surface temperature of the fixing belt may not be controlled properly. Moreover, when the surface temperature of the fixing belt is not controlled with accuracy, heat is not adequately supplied to an unfixed toner image carried on a recording medium. This results in deterioration of image quality.
According to one aspect of the present invention, a fixing device for fixing a toner image on a recording medium includes a heating roller including a heat source, a fixing roller, an endless fixing belt extended around at least the heating roller and the fixing roller and configured to be heated by the heating roller to fuse a toner image on the recording medium, a pressing roller disposed opposite to the fixing roller via the fixing belt and configured to press the recording medium against the fixing roller, a temperature detecting member configured to detect a surface temperature of the heating roller, and a control device configured to control a surface temperature of the fixing belt at a predetermined temperature based on a surface temperature of the heating roller detected by the temperature detecting member. The temperature detecting member contacts a circumferential surface of the heating roller where the fixing belt is not extended around, such that the temperature detecting member is held in a low frictional relation to the circumferential surface of the heating roller.
According to another aspect of the present invention, a diameter of a circumferential surface of the heating roller where the temperature detecting member contacts is smaller than a diameter of the other circumferential surface of the heating roller.